1. Field of the Invention
The present invention relates to a lamp, and more particularly to a lamp driving device and a driving method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing erroneous driving caused by the surrounding environment while observing safety standards.
2. Description of the Related Art
Among display devices for displaying images, a liquid crystal display (“LCD”) device has had a broad scope of application because it is light, thin, and consumes little power. For example, LCD devices are used in office automation equipment, audio/video equipment, etc. In the LCD device, a desired picture is displayed on an LCD screen by controlling the amount of transmitted light through the LCD device in accordance with a video signal applied to a plurality of control switches forming a matrix.
In general, the LCD device is not a self luminous display device because it requires a separate light source, such as a backlight. The backlight for the liquid crystal display device can be a direct type or an edge type depending on the disposition of a lamp in the backlight. In the edge type backlight, a lamp is installed at an outer edge of a flat LCD panel of the LCD device and light propagates from the lamp through a transparent light guide panel to be incident onto the entire back surface of the LCD panel. In the direct type backlight, a plurality of lamps is disposed on a plane. A diffusion plate is installed between the lamp and the LCD panel to maintain a fixed gap between the LCD panel and the lamp.
The backlight can also be classified as a cold cathode fluorescent lamp type or an external electrode fluorescent lamp type depending on the type of the lamp used on the backlight. The cold cathode fluorescent lamp is formed of a glass tube and power is supplied to the lamp by inserting electrodes through both ends of the glass tube. The external electrode fluorescent lamp is also formed of a glass tube, but power is supplied to an electrode part formed of a metal material covering both ends of the glass tube.
FIG. 1 is a schematic diagram of a lamp driving device in accordance with the related art. Referring to FIG. 1, the lamp driving device 60 is connected to a plurality of lamps 36 and includes a switch circuit 46, such as an inverter, which converts a DC power Vin received from an external power source to into an AC signal. A transformer 48 is provided to boost the voltage of the AC signal generated by the inverter 46 and supplies the boosted AC signal to the lamps 36. A feedback circuit 42 is provided for detecting a current supplied to the lamp 36 by the inverter 46. A controller 44 of the lamp driving device 60 controls the inverter 46 in accordance with a feedback signal generated by the feedback circuit 42. The transformer 48 includes a primary winding 51 connected to the inverter 46; a secondary winding 53 which is synchronized with the primary winding 51 to generate an alternating current; and an auxiliary winding 52 disposed between the primary and secondary windings 51 and 53.
The lamp driving device 60 having such a structure should satisfy the safety standards with respect to a user's safety. The safety standards require that the current flowing through a user should be restricted to a current in milliamps (mA) of less than or equal to 0.7 fold of a system operation frequency in KHz when the user is in contact with the lamp driving device 60. To test compliance with the standards, a test lamp is fabricated based on the safety standards. For example, a no-load condition of about 2 KΩ is shown in FIG. 1 as a contact resistor 59, which corresponds to a contact between the user and the lamp driving device 60, and a normal resistance component of the lamp 36 is about 200 KΩ. When the lamp 36 is operated normally, a usable frequency is about 65 KHz and a voltage of the secondary winding 53 is about 1500V.
A resonance characteristic of the secondary winding 53 changes rapidly when the non-load value of 2 KΩ is in contact with the secondary winding 53. Generally, the secondary winding 53 is resonant in parallel, and a voltage gain of an input and an output in the parallel resonance is changed in proportion to the resistance component of a load. Thus, an equivalent resistance component 200 KΩ of the lamp 53 and the 2 KΩ no-load resistor 59 are connected in parallel so that the equivalent resistance appearing in the secondary winding 53 is about 22 KΩ (200 KΩ/2 KΩ) to generate a load change of about 1/100. Thus, a gain change of about 1/100 is also generated in the voltage of the secondary winding 53 to satisfy the safety standards. Quantitatively, a lamp safety standards limited current of a 65 KHz frequency usage is 46 mA (0.7×65). Further, the voltage of the secondary winding 53 is about 15V (1500× 1/100) since the gain is 1/100, and the current passing through 2 KΩ becomes 7 mA according to Ohm's law, thereby satisfying the 46 mA upper limit required by the safety standards. A driving device is required that does not violate the safety standard for a user.
When driving a plurality of lamps 36, e.g., in case of driving ten lamps 36, the equivalent resistance of the lamp 36 becomes 20 KΩ, and at this moment, if the user is connected to the system, i.e., 2 KΩ resistance is connected as the no-load resistor 59 of the system, the gain of the output voltage becomes 1/10. Accordingly, the voltage of the second winding 53 becomes about 150V so that the current flowing in the no-load resistor 59 becomes 70 mA, thereby violating the safety standards.